Catalytic gas-phase oxidation reactions using fixed bed reactors are widely carried out in the field of petrochemical industry. Various oxygen-containing organic compounds, or oxidized organic compounds, which are useful as reaction feedstocks, etc. are produced as chemical products by oxidizing starting organic compounds. Examples of the many reactions of this type carried out include the production of ethylene oxide by the catalytic gas-phase oxidation of ethylene, the production of (meth)acrylic acid by a two-stage oxidization using propylene, isobutylene, tert-butanol and/or methyl tert-butyl ether as the starting material, the production of phthalic anhydride by the catalytic gas-phase oxidation of o-xylylene and/or naphthalene, and the production of maleic anhydride by the catalytic gas-phase oxidation of benzene or n-butane. Of these compounds manufactured by catalytic gas-phase oxidation, (meth)acrylic acids are industrially important substances as feedstocks for various types of synthetic resins, paints and plasticizers. Acrylic acid in particular is a leading compound which is produced and supplied by catalytic gas-phase oxidation; it has become even more important recently as a precursor of water-absorbing resins, and demand is on the rise.
As prior-art (meth)acrylic acid production methods, a two-stage catalytic gas-phase oxidation process which uses propylene, isobutylene, tert-butanol and/or methyl tert-butyl ether as the starting material to form (meth)acrolein in a first-stage catalytic gas-phase oxidation reaction, and converts the resulting (meth)acrolein into (meth)acrylic acid in a second-stage catalytic gas-phase oxidation reaction is the most common, and in wide use industrially. This process exists in, broadly speaking, two forms, for example, in cases where acrylic acid is produced: a process wherein a feedstock gas containing propylene and molecular oxygen is converted to acrolein by catalytic gas-phase oxidation in a first fixed-bed reactor packed with a catalyst for converting propylene to acrolein (referred to below as the “first-stage catalyst”), following which acrylic acid is produced by catalytic gas-phase oxidation in a second fixed-bed reactor packed with a catalyst for converting the resulting acrolein to acrylic acid (referred to below as the “second-stage catalyst”); and a process wherein acrylic acid is produced by the catalytic gas-phase oxidation of a feedstock gas containing propylene and molecular oxygen in a single reactor which is a fixed-bed reactor packed with the first-stage catalyst and the second-stage catalyst.
Various other processes have been under investigation in recent years, including a process wherein propane, which is cheaper than propylene, is used as the feedstock and converted to propylene by dehydrogenation or oxidative dehydrogenation, and the resulting propylene is subjected to the above-described two-stage catalytic gas-phase oxidation; a process wherein propane is converted directly to acrylic acid by catalytic gas-phase oxidation in a single step; and a process which, owing to concerns over future resource depletion and increased carbon dioxide in the atmosphere, involves using as the starting material plant-based glycerin for which there is no concern of resource depletion because the source of carbon is carbon dioxide in the atmosphere, and which substantially do not contribute to increased carbon dioxide in the atmosphere, converting the glycerin to acrolein by a dehydration reaction, then producing acrylic acid from the resulting acrolein by catalytic gas-phase oxidation.
When employing such a catalytic gas-phase oxidation reaction on an industrial scale, the reaction is generally carried out continuously for a long period of time, although the reaction must sometimes be stopped for periodic inspection of the production equipment or emergency shutdown for the sake of safety when abnormal reaction occurs. Various art has been proposed concerning catalytic gas-phase oxidation reaction stopping methods or start-up methods to enable the long-term stable and safe production of the target product in a high yield. For example, when restarting the reaction following shutdown for periodic inspection or the like, there are cases where, due to, for example, the state in which the catalyst is held during the shutdown period, the catalyst performance decreases or takes a long time to recover to the same level as that prior to shutdown, which has a large impact on productivity. Hence, a major challenge is how to minimize production losses when the reaction is started up again from a shut-down state. A number of technical innovations relating to this have been described in the art.
Several innovations relating to the production of (meth)acrylic acids have been proposed. For example, it has been disclosed that, by supplying an oxygen-containing gas to the reactor, degradation of the catalyst performance can be prevented even during shutdown of the catalytic gas-phase oxidation step. Specifically, it has been reported that when operation of the catalytic gas-phase oxidation step is stopped and up until operation is started up once again, unless oxygen is continuously supplied while maintaining the catalyst temperature, reducing substances such as by-products—some of them heavy—which have accumulated on the catalyst reduce the catalyst, thereby changing the oxidation state of the catalyst and causing the catalyst performance to deteriorate; and that, for this reason, if molecular oxygen is supplied during shutdown as well, the catalyst oxidation state is maintained and the catalyst performance does not degrade (see, for example, Patent Document 1). However, in the examples described therein, desirable effects occurred only when a fresh catalyst not used to the reaction was packed into the reaction tube and an oxygen-containing gas is supplied prior to the operation; in actual production, no evaluations during reaction shutdown, such as a periodic inspection or emergency shutdown, were carried out whatsoever on the catalyst after it had been used to the reaction. Thus, in cases where oxygen was continuously supplied while maintaining the catalyst temperature during shutdown after the catalyst had been used to the reaction, catalyst oxidation proceeded, disrupting the subtle oxidation state of the catalyst at which a high yield was obtained in a steady state, and causing the yield to decline until a steady state was reached after the reaction was restarted. In stable production over an extended period of time, this can hardly be regarded as satisfactory.
In addition, a process has been disclosed for, with the exception of emergency shutdown in which operation is pointless, reliably stopping the reaction only when shutdown is necessary by shutting down the operation only in cases where both concentration values for each gas obtained by calculations based on the flow rates of each of the introduced gases at the reactor inlet and analytical values obtained by gas analyzers fall outside of specified ranges (see, for example, Patent Document 2). However, this art relates to a method which is able to avoid emergency shutdown due to, for example, analyzer malfunction and carry out emergency shutdown only when necessary; it discloses nothing concerning the impact on catalyst performance during shutdown for periodic inspections and the like.
In addition, it has been disclosed that stable reactor startup can be achieved by avoiding the explosive range that arises depending on the composition of the feedstock to be oxidized and the molecular oxygen-containing gas which is fed to the reactor, and by reducing the feed rate of dilution gas (see, for example, Patent Document 3). However, this art relates only to a method for efficiently starting up a reaction from a shutdown state. As with Patent Document 2, nothing whatsoever is disclosed concerning the impact on catalyst performance during shutdown following continuous operation.    Patent Document 1: Japanese Patent Application Laid-open No. 2005-314314    Patent Document 2: Japanese Patent Application Laid-open No. 2004-277339    Patent Document 3: Japanese Patent Application Laid-open No. 2002-53519